Loop Mail is the USPS's internal term for mail that goes out for delivery and is returned by the carrier because it was determined that the delivery point assigned by the OCR engine is not the one intended by the sender. Loop mail pieces can be caused by OCR errors as well as by incomplete, ambiguous, or invalid destination addresses.
Currently, all loop mail is sorted by hand, which is a very expensive process. This is done because of the possibility that a loop mail piece that is reprocessed by an OCR engine will again be assigned to the same incorrect address, and thereby continue to loop repeatedly through the system.